Line (X) "A Unit of Models with this Special Rule can control Objectives more easily and scores more Victory Points form controlling an Objective."
If a Unit that Controls an Objective includes a majority of Models with the Line (X) Special Rule then, whenever the Controlling Player scores Victory Points for Controlling that Objective, an additional number of Victory Points equal to the value of X are scored. For example a Unit of 10 Models of which the majority have the Line (3) Special Rule, hold an Objective worth 1 Victory Point when the Controlling Player scores Victory Points for that Objective, they would score a total of 4 Victory Points.
Furthermore, the value of X is added to the Tactical Strength of the Model that has that Special Rule to determine its ability to Control Objectives (see Core Missions in the Warhammer: The Horus Heresy – Age of Darkness Rulebook).